I made a pull request for that here (https://github.com/opnsense/core/pull/1822). Please note that the default URL was never intended to be used for production. It was more a sample for a user how to input the URL as well as an requirement by the framework (the field must be set with something).

Note that when I wrote the configuration page for ICAP, the c-icap plugin did not exist. I worked with my own ICAP services which did not have default URLs.

Now, thanks to Michael, we have a plugin for that and I wrote the docs as early as possible as this is a frequently requested feature and the URLs were not documented somewhere else for OPNsense.